A natural gas extracted from subsoil is, under normal conditions of temperature and pressure, a mixture of gaseous hydrocarbons. Typically, a natural gas is, for example constituted of 75% methane, 20% other gaseous hydrocarbons, dominantly ethane, and 5% acid gases, namely carbon dioxide (CO2) and hydrogen sulfide (H2S). Liquefied Petroleum Gas (GLP) are generally mainly formed of three- or four-carbon-chain gaseous hydrocarbons, i.e. propane, butane and their unsaturated versions propene and butene. Accompanying these components are traces of contaminants, essentially sulfurous compounds, namely sulfur carbonyl (COS) and mercaptans. The mercaptans are principally divided into methyl-mercaptan (CH3SH), ethyl-mercaptan (C2H5SH), propyl-mercaptan (C3H7SH) and possibly higher molecular weight mercaptans.
The more acidic a natural gas is, that is to say the more carbon dioxide and hydrogen sulfide it contains, the higher is its content of sulfurous compounds and consequently mercaptans. In certain natural gas deposits the mercaptans content can therefore exceed the limit tolerated for a commercial natural gas. Therefore, whether the gas is to be sold in gas or liquid form, the mercaptans must be extracted.
Because the gases containing the mercaptans are acidic, they are subjected to a first step of de-acidification in order to extract H2S and CO2. However, the mercaptans are only slightly extracted when classical processes of de-acidification, that are most often washings with amine solutions, are used. It is thought that barely not more than one third, if not one quarter, of mercaptans present in natural gas is absorbed in this way. Their extraction necessitates, therefore, a supplementary extraction. Two types of treatment are commonly used today: adsorption by a molecular sieve or cryogenic condensation.
U.S. Pat. No. 5,291,736 and U.S. Pat. No. 5,659,109 indicate that the cryogenic condensation of LPG is accompanied by that of the mercaptans. The mercaptans are then found concentrated in the condensed liquids.
The article <<Gas processing options for mercaptans and carbonyl sulfide removal from NG and NGL streams>> (UOP, AIChE 1993, Spring National Meeting, Houston, Tex., Mar. 28 to Apr. 1, 1993) shows flow sheets of three plants, the first of which (plant A—FIG. 1) is a liquefaction plant for gases with a high sulfurous compound content. It indicates that the LPG products are highly contaminated by the simultaneously condensed mercaptans and that the latter concentrate naturally in the propane and the butane. The mercaptan content measured in the LPG reaches levels of 112 and 288 ppm by weight in the propane and in the butane, respectively. These commercially unacceptable levels make necessary the treatment of the LPG.
Irregardless of the origin of the condensed hydrocarbons—extraction from a subsoil natural gas or from a refinery gas, the cryogenic condensation during the cooling producing the liquefaction of the natural gas—the hydrocarbon liquid mixture is essentially made up of ethane mixed with heavier hydrocarbons. It is observed that the methyl- and ethyl-mercaptans concentrate preferentially in the butane and the propane while the propyl-mercaptans and the heavier mercaptans stay in the condensates. The following description focuses on the LPG butane and propane cuts, but the process according to the invention is also applicable to all cuts (for example condensate) as long as their density permits treatment by washing with sodium hydroxide (see below). The sulfur content in the butane and propane is therefore high, frequently greater than 1000 ppm, if not greater than 1%.
At such high content levels, mercaptan extraction from the propane and butane cuts cannot be done using molecular sieves. It is carried out by washing with sodium hydroxide, an example of which is given for methylmercaptan:2CH3SH+2NaOH→2CH3SNa+2H2O
The regeneration of the sodium hydroxide solution with oxygen transforms the mercaptans into disulfide.2CH3SNa+½O2+H2O→CH3SSCH3+2NaOH
The general reaction can be written:2CH3SH+½O2→(CH3S)2+H2O
Two methylmercaptan molecules give one dimethyldisulfide molecule. The reaction is similar for the other mercaptans. The mixture of disulfides obtained from the mercaptans according to this reaction is known as Disulfide oil (DSO).
To get rid of the DSO, the most standard practice consists of mixing it with hydrocarbon cuts (condensates, naphtha or others) to be treated afterward in the refinery. However, mainly in gas treatment plants, it happens that such cuts are not available, thus making then necessary the treatment of DSO in situ.
A practical way to eliminate the DSO is to treat it by oxidation jointly with H2S, in a Claus reaction-based sulfur recuperation unit according to the following reaction for, given as an example, dimethyldisulfide:(CH3S)2+1½O2→2CO2+2SO2+3H2O
However, for the reaction to reach completion it must be carried out in the presence of an excess, with respect to the stoichiometry, of oxygen, whereas the H2S oxidation reaction in a Claus unit takes places in the absence of oxygen. The quantity of DSO that is possible to incinerate jointly with H2S is therefore limited and often inferior to that produced during the treatment of LPG. Today this method has not yet been used industrially.
Another way to eliminate the DSO is to incinerate it, outside of a Claus unit. For total combustion to occur, it must be carried out in an excess of air. The smoke resulting from the combustion contains sulfur dioxide, SO2. It can be introduced into the Claus unit but the residual oxygen still present in the smoke must be first separated. It is thus necessary to wash the smoke with a physical solvent that separates the sulfur dioxide from the residual oxygen and concentrates the former before injection into the Claus unit. This technique, however, presents the inconvenience of operating in a very corrosive medium and of requiring the use of noble metallurgical products for the equipment, for example, stainless steel. An industrial application of this process is being carried out in the Dolphin treatment plant, a plant fed by North Dome natural gas
The invention, contrary to the two aforementioned processes, relates to the treatment of DSO integrated into the whole sulfur treatment chain of a gas treatment plant.